The present invention relates in general to the processing of bulk unvulcanized rubber material, and in particular to a bale processor for cutting or dicing a bale or slab of unvulcanized elastomer such as ethylene propylene diene terpolymer (EPDM) or styrene butadiene (SBR).
Bulk synthetic rubber such as unvulcanized elastomer is normally supplied in a dense rubber bale or slab, typically 24xe2x80x3xc3x9718xe2x80x3xc3x978xe2x80x3 size and 24 kg weight, often wrapped in a thin protective plastic film. Due to its high bulk density and compact size, the bulk rubber bale or slab is the most economical and safe form for shipping, storage and handling.
The term rubber as used herein refers to a natural rubber or polymer resin having in its unvulcanized state properties of deformation upon stress and recovery upon release of the stress. A rubber can be further defined as having a glass transition temperature of below 20xc2x0 C. Most rubbers have a raw polymer Mooney value of from about 20 to about 125 measured at 100xc2x0 C. (212xc2x0 F.) after 4 minutes using a large rotor, i.e. a ML-4 reading, and have an elongation at break of from about 100 percent to about 1000 percent or more.
Examples of unvulcanized bulk rubbers that can be processed by the present invention include natural rubber, polyvisoprene rubber, polybutadiene rubber, cis-polybutadiene rubber, polychloroprene rubber, polysulfide rubbers, polypentenamer rubbers, polyacrylated rubbers, poly(butadieneacrylonitrile) rubbers, poly(isopreneacrylonitrile) rubbers, poly(styrenebutadiene) rubbers, poly(isoprene-styrene rubbers) poly(ethylene-propylene-diene) rubbers, and the like. The term rubber as used in this invention also includes blends of two or more of the elastomers. The rubbers may be blended with resins or fillers prior to forming the bale or slab.
In recent years, the use of thermoplastic elastomers (TPE), which are melt-mixed blends of thermoplastic resins such as polypropylene and synthetic elastomer, is increasing rapidly. Blends of thermoplastic resin, elastomers, plasticizers or softeners, fillers and stabilizers offer significant advantages over thermosetting elastomers, including 100% recyclability, ready-to-use pelletized form, no need for curing, lower density, ease of processing, lower cost per unit, and colorability.
Thermoplastic elastomers are produced using either an internal batch mixer or continuous mixers. In recent years, many producers of TPEs have used continuous mixers because of their ability to provide uniform product quality, short residence time and versatility. Various ingredients are metered directly through small input openings in the continuous machine using automatic feeding devices. For consistent feeding and trouble-free operation, all ingredients must be small in size, uniform in shape and non-agglomerating in nature. Since a rubber bale is very large, it must be reduced to pieces or fragments that are size-compatible with automatic feeding equipment and other ingredients. Even in a batch mixer, where whole dense bales can be used, smaller size feedstock reduces cycle time and hence reduces overall productivity and quality of product. In making rubber-based adhesives, smaller size rubber feedstock enhances the rate of solvent diffusion.
Various devices including guillotine cutters, granulators and shredders use rotary knives, shears or saw blades for comminuting and reducing the size of scrap plastic and rubber. For example, U.S. Pat. No. 4,280,575 discloses a machine for cutting and metering a slab of unvulcanized rubber, which utilizes a continuous blade band sawing machine for cutting slices of rubber. U.S. Pat. No. 4,929,086 discloses a shredding machine which uses a rotary screw blade equipped with both radial and longitudinal knives for cutting shreds of polymer from a feedstock bale.
Such machinery is not suitable for dense bales of rubber because (1) unvulcanized rubber tends to flow under the influence of shear; (2) such machines are large in size, require special installation, use large amounts of energy, create loud noise, break down frequently, and require time-consuming cleaning; and, (3) the resulting product is either very large in size (e.g. as produced by guillotine cutters) or consists of a mixture of fine powder, fluff and large irregularly shaped chunks that are not suitable for continuous feeding applications. Moreover, the reduced material tends to stick and agglomerate, and has limited shelf life. Such machines are intended for large scale operation in production environment only and not suitable for small scale operations (i.e. lab scale devices).
Some producers of thermoplastic elastomers use a two-step method in which elastomer bale material is mixed with thermoplastic resin using an internal mixer, and reduce the size of the mixed material into pellets using an extruder-pelletizer or dices using a roll mill-dicer. Besides being a costlier process, there are other limitations to that conventional process: (1) the rubber material is subjected to two heat and shear steps which affects its durability; (2) many high molecular weight elastomers are highly oil extended which requires long mixing times; (3) are applicable only where the formulation consists of a large amount of thermoplastic resin; (4) the resulting pellets or dice must be dusted with a partitioning agent to keep them from re-agglomerating during handling; and, (5) such pelletized materials have short shelf life and tend to agglomerate when stored under hot and humid conditions.
Some producers of elastomers provide rubber bales in form which can easily be broken into small popcorn-like crumbs. Even though very beneficial, such feed stock also has significant limitations: (1) crumbs with irregular surfaces tend to have very low bulk density and do not feed well using conventional feeders; (2) the crumbs tend to interlock in the feed hopper causing feed-blocking; (3) the crumbs do not pack efficiently and thus require large storage space; (4) only those elastomers with medium molecular weights, high comonomer content and no oil are available in the form of dense bales; and (5) adding oil during mixing reduces shear, prolongs mixing time, and thus reduces production rates.
Most recently, some producers using new catalyst technology are supplying selected grades in free-flowing granular or large pellet forms. Currently, only a small range of some selected elastomers are available in the free-flowing granular shape, and none with any oil.
From the above discussion, it is clear that the baled elastomer must be reduced in size, preferably to portions of uniform size and shape to accommodate the needs of continuous mixing processes. The conventional reduction methods discussed above have one or more of the following limitations:
(1) high cost of size reduction equipment;
(2) irregular shape and size of resulting product not suitable for continuous feeding;
(3) lower bulk density of reduced product requires larger storage area;
(4) limited shelf life;
(5) requires unwanted partitioning agents to extend shelf life; and,
(6) size reduction method poses limitations on choice of elastomer and mixing method.
Small cubes or blocks of a predetermined size and uniform shape are reduced from a bale or slab of unvulcanized rubber for continuous feeding at a controlled rate into a mixing machine or blender along with compounding chemicals during the mixing and extrusion of synthetic rubber and elastomeric products. A bale or slab of unvulcanized rubber is advanced along a loading platform on the input end of a processor console. The bale is fed incrementally into a first cutter assembly at a first cutter station where a segment of predetermined width is sliced from the leading end of the bale. The segment is transferred by a vacuum pick-up head to a second cutter station where it is secured for further reduction on a vacuum hold-down table.
After the segment is immobilized on the hold-down table, it is then sliced into elongated, parallel strips by an X-axis cutter head which includes an array of rotary cutter blades that are extendable and retractable across the segment in parallel with the X-axis. While the reduction strips are firmly held in place on the vacuum hold-down table, they are diced by a Y-axis cutter head which includes an array of rotary cutter blades that are extendable and retractable across the elongated strips in parallel with the Y-axis.
The slab segment is thus reduced to multiple cubes of predetermined length, height and width dimensions as established by the initial segment slice dimension and by the spacing of the roller cutting blades in the X-cutter head and the Y-cutter head, respectively. The bale is advanced incrementally at the speed demanded by the blending process, so that feed stock cubes are continuously transferred at a controlled rate to the feed throat of a mixing or shaping machine such as an extruder or internal mixer.